摘要
本文探讨介质环境性质对萘酚的酸碱平衡及荧光光谱的影响。在弱酸性水溶液中,1-萘酚吸光受激成为ROH~*后,在其寿命期内,只有经酸式离解成为RO^(-*)的那部分,能发荧光,而未离解的ROH~*则被猝灭而不发光。在乙醇介质中,由于乙醇接受质子能力较弱,1-萘酚的激发单线态ROH~*不离解,只观察到ROH~*所发的光,看不到RO^(-*)所发的荧光。萘酚在β-CD溶液中,由于发生包结作用,则既可看到萘酚分子型体的受激发光,也能观察到离子型体所发的光。但1-萘酚和2-萘酚的荧光性质有较大差异,β-CD浓度对它们的荧光强度的影响有明显的差别。基于β-CD对1-萘酚及2-萘酚的酸碱平衡及荧光光谱的不同效应,本文提出萘酚是以轴向形式包结进入β-CD空腔的。
The effect of media properties on the acid-base equilibrium and fluorescence spectra of naphthols was studiedo In weak acidic aqueous solution(pH5.8, the pKa of 1-naphthol is, equal to 9.45), although 1-naphthol predominantly exists in the ground state molecular form (ROH), an optically excited 1-naphthol molecule(ROH~*) will either dissociate during its lifetime into its excited conjugated base (RO^(-*) to fluoraseence(λ_(em)=466nm), or be quenched by the solvent molecules if remains in its acidic form (ROH~*). In 1-naphthol aqueous solution(pH5.8), only its emission band at 466 nm was observed and ascribed to RO^(-*). In ethanol, however, only the fluorescence of ROH~* can be observed whereas Virtually no RO^(-*) exists due to the weak proton-susceptibility of ethanol. In aqueous β-cyclodextrin (β-CD) solution, because of the inclusion of 1-naphthol molecule into β-CD, fluorescence from both RO^(-*)(λ_(em)=466nm)and ROH~*(λ_(em)=362nm) can be observed. The excited singlet state of 1-naphthol is somewhat protected from dissociation and quenching after inclusion into the β-CD cavity, which has a size compatible with the size of the naphthol molecule, and an enhancement of I_F of 1-naphthol could be induced by the micro-environment properties of β-CD cavity. Nevertheless, the fluorescence behaviors of 1-naphthol are different from those of 2-naphthol in β-CD soluticn in term of the depertdence of fluorescence intensities on the β-CD concentrations. According to the speotrophotometric studies of naphthol aqueous solution, β-CD apparently shifted the acid-base equilibrium of 1-naphthol. The pKa of 1-naphthol was shifted from 9.45 to 10.20 due to the presence of β-CD, with △pK=0.75. 2-naphthcl showed a similar but smaller shift in equilibrium in the presence of β-CD, with △pK=0.17. The influence of β-CD on the acid-base equilibrium of 1-naphthol is stronger than that of 2-naphthol. Base on the relative susceptibility of 1-naphthol and 2-naphthol to the effect of β-CD, we conclude that the inclusion of naphthol molecules within the β-CD cavity is axial rather than equatorial.
出处
《分析化学》
SCIE
EI
CAS
CSCD
北大核心
1992年第9期1013-1017,共5页
Chinese Journal of Analytical Chemistry
基金
国家自然科学基金
关键词
Β-环糊精
萘酚
酸碱平衡
荧光光谱
Media microenvironmental properry
β-Cyclodextrin
1-Naphthol
Acid—base equilibrium
Flucrescence spectrum.
